The r->request_body_no_buffering flag was introduced.  It instructs
client request body reading code to avoid reading the whole body, and
to call post_handler early instead.  The caller should use the
ngx_http_read_unbuffered_request_body() function to read remaining
parts of the body.

Upstream module is now able to use this mode, if configured with
the proxy_request_buffering directive.

If the last header evaluation resulted in an empty header, the e.skip flag
was set and was not reset when we've switched to evaluation of body_values.
This incorrectly resulted in body values being skipped instead of producing
some correct body as set by proxy_set_body.  Fix is to properly reset
the e.skip flag.

As the problem only appeared if the last potentially non-empty header
happened to be empty, it only manifested itself if proxy_set_body was used
with proxy_cache.

LibreSSL removed support for export ciphers and a call to
SSL_CTX_set_tmp_rsa_callback() results in an error left in the error
queue.  This caused alerts "ignoring stale global SSL error (...called
a function you should not call) while SSL handshaking" on a first connection
in each worker process.

Keeping the ready flag in this case might results in missing notification of
broken connection until nginx tried to use it again.

While there, stale comment about stale event was removed since this function
is also can be called directly.

Repeatedly calling ngx_http_upstream_add_chash_point() to create
the points array in sorted order, is O(n^2) to the total weight.
This can cause nginx startup and reconfigure to be substantially
delayed.  For example, when total weight is 1000, startup takes
5s on a modern laptop.

Replace this with a linear insertion followed by QuickSort and
duplicates removal.  Startup for total weight of 1000 reduces to 40ms.

Based on a patch by Wai Keen Woon.